In a typical electrostatographic reproducing apparatus, a light image of an original to be copied is recorded in the form of an electrostatic latent image upon a photosensitive member, and the latent image is subsequently rendered visible by the application of a thermoplastic resin toner powder. The visible toner image is initially in a loose powdered form that can be easily disturbed or destroyed but is usually fixed or fused on a receiver, which may be, for example, plain paper.
In order to fuse the toner particle image onto a receiver surface permanently by heat, it is necessary to elevate the temperature of the toner particles to a point at which they coalesce and become tacky. This heating causes the toner to flow to some extent into fibers or pores on the receiver surface. Thereafter, as the toner material cools, its solidification causes it to be firmly bonded to the receiver surface.
Typically, thermoplastic resin particles are fused to the substrate by heating, generally to a temperature of about 90.degree. C. to 160.degree. C., but sometimes higher, depending on the softening range of the particular resin used in the toner. It is not desirable, however, to exceed a temperature of about 200.degree. C. because of the tendency of the receiver to discolor at such elevated temperatures, particularly if it includes a paper substrate.
Several approaches to thermal fusing of toner images have been described in the prior art, including the substantially concurrent application of heat and pressure. This may be achieved by, for example, a pair of rollers, a fuser roller and a pressure roller that are maintained in pressure contact, a fuser plate or belt member in pressure contact with a pressure roller, and the like. Heat may be applied to one or both of the rolls, plates, or belts. The fusing of the toner particles takes place when the proper combination of heat, pressure and contact time are provided. The balancing of these parameters to bring about the fusing of the toner particles is well known in the art and can be adjusted to suit particular machines or process conditions.
During operation of a fusing system in which heat is applied to cause thermal fusing of the toner particles onto a support, both the toner image and the receiver are passed through a nip formed between the roller pair, or between the pressure roller and fuser plate or belt member. The concurrent transfer of heat and the application of pressure in the nip effects the fusing of the toner image onto the receiver. It is important in the fusing process that no offset of the toner particles from the support to the fuser member take place during normal operations. Toner particles offset onto the fuser member may subsequently transfer to other parts of the machine or onto the receiver in subsequent copying cycles, thereby increasing the background or interfering with the material being copied there. "Hot offset" occurs when the temperature of the toner is raised to a point where the toner particles liquefy during the fusing operation, and a portion of the molten toner remains on the fuser member. The extent of hot offset is a measure of the release property of the fuser roll; accordingly, it is desirable to provide a fusing surface having a low surface energy to enable the necessary release.
For further improvement in the release properties of the fuser member, it is customary to apply release agents to the fuser member surface to ensure that the toner is completely released from the surface during the fusing operation. Typically, release agents for preventing toner offset are applied as thin films of, for example, silicone oils. U.S. Pat. No. 3,810,776 describes a release agent of a low viscosity silicone oil in which is dispersed a high viscosity component such as zinc or aluminum stearate or behenate. Polyorganosiloxanes containing various functional groups that interact with a fuser member surface are well known in the art. For example, mercapto-functionalized polyorganosiloxanes are disclosed in U.S. Pat. No. 4,029,827, and analogous amino-functionalized materials are described in U.S. Pat. Nos. 5,512,409 and 5,516,361. Silicone release oils containing other functional groups such as carboxy, hydroxy, epoxy, and isocyanate are described in U.S. Pat. Nos. 4,101,686 and 4,185,140.
In a fusing system including a nip formed by a pair of rollers, the pressure roller is commonly provided with a surface layer, or sleeve, of a fluorocarbon plastic such as, for example, a perfluoroalkoxy (PFA) polymer, a fluoroethylenepropylene (FEP) polymer, or a tetrafluoroethylene (TFE) polymer over a more resilient blanket layer such as, for example, a silicone rubber. The surface of the fuser roller, which is often but not necessarily more resilient than the pressure roller surface, may comprise, for example, a silicone rubber or a fluoroelastomer.
Regardless of the materials employed, contact between the roller surfaces during passage of a toner image receiver, usually paper, through the nip causes an electrostatic charge to build up on the fuser roller surface. The magnitude and polarity of the electrostatic charge depends at least in part on the relative position of the pressure and fuser roller surface materials in the triboelectric series. In L. B. Schein, Electrophotography and Development Physics, 2nd edition, Springer-Verlag, New York, 1992, page 78, is presented a triboelectric series table showing a silicone elastomer with silica filler at the extreme positive end of the series and polytetrafluoroethylene at the extreme negative end.
Generation of an electrostatic charge at the roller nip may, depending on the magnitude and polarity of the charge on the fuser roller surface and the surface charge properties of the toner composition particles employed, result in serious problems of toner offset or paper jamming, or both. It is therefore desirable to prevent or suppress the buildup of static charge at the nip to keep it at a very low level, ideally zero.
U.S. Pat. No. 4,970,559 describes a mixture for forming a roller layer that comprises an organic polymer and an inorganic fine powder carrying an absorbed liquid antistatic agent. In commonly assigned U.S. Pat. No. 5,735,945, a static charge-suppressing release agent for pressure and fuser rollers is described. A problem with using static-charge suppressing release agents is that they have to be continuously applied in the correct amounts. If an incorrect amount of release agent is applied image artifacts can result.